A primary purpose of a vehicle's suspension system is to provide vertical compliance between the road and the chassis, in order to isolate the chassis occupants from the roughness in the road and to maintain tire contact with the road, thus providing a path for transferring forces from the bottom of the tire to the chassis, such as to change the speed or direction of the vehicle. Examples of some common independent suspension linkages are known generally as strut & link (also called MacPherson strut), double A-arm (also called double wishbone or SLA), semi-trailing arm, and multi-link.
Each wheel assembly is connected to the chassis by one or more links. A link is defined as a substantially rigid member with a joint or joints at each end that allows a particular motion to take place. It is these links that control the motion (or path) of the wheel as it moves up and down over road bumps. These links also have to transmit the forces generated at the tire-road interface to the chassis. Rubber bushes are typically used at the ends of links to reduce the transmission of vibrations to the chassis. Unfortunately, the use of bushes also introduces compliance into the linkage that can be undesirable for accurately controlling wheel motion.
In an active suspension, controlled forces are introduced to the suspension, such as by hydraulic or electric actuators, between the sprung mass of the vehicle body and its occupants, and the unsprung mass of the wheel assemblies. The unsprung mass is the equivalent mass that reproduces the inertial forces produced by the motions of those parts of the vehicle not carried by the suspension system. This primarily includes the wheel assemblies, any mass dampers associated with the wheel assemblies, and some portion of the mass of the suspension links. The sprung mass is the mass of those parts of the vehicle carried by the suspension system, including the body. Active suspension systems preferably are able to introduce forces that are independent of relative wheel motions and velocities.
U.S. Pat. No. 4,981,309 discloses an active suspension system employing electro-magnetic actuators at each wheel assembly of a rolling vehicle. U.S. Pat. No. 6,364,078, and EP publication 0982162, published Mar. 1, 2000, together disclose a mass damper useful with such electromagnetic suspension actuators and that can move independent of the wheel assembly, but only in a substantially vertical direction. In all other directions, the mass of the mass damper is effectively added to the inertia of the unsprung mass. The entire contents of the above U.S. patents are incorporated herein by reference as if set forth in their entirety.
Generally, all kinematically-induced wheel forces are either forces created by the interaction between the tires and the road, or inertia forces generated by the motion of the unsprung mass. The forces occurring between the tires and road are transferred via the suspension system to the body. As long as the wheel assembly does not change its horizontal position or angular orientation relative to a smooth road surface, no substantial lateral or longitudinal tire forces (ignoring friction) will be created. Unfortunately, most practical independent suspension linkages do not provide pure vertical wheel motion, and thus some horizontal movement of the tire is inevitable.
It is both unnerving and disconcerting to feel a steering yaw motion induced without any steering input from the driver. Self-steering, as it is generally called, is especially objectionable when it occurs in the rear of the vehicle, behind where the passengers are seated. The static toe angle of a wheel, measured at a specific height of the wheel relative to the chassis, is the angle between the central longitudinal axis of the vehicle and the line intersecting the center plane of one wheel with the road surface. A wheel is “toed-in” if the forward portion of the wheel is tuned toward the vehicle's central longitudinal axis, and “toed-out” if turned away. It is desirable that the static toe angle be very close to zero degrees at speed, to reduce tire wear and rolling resistance. It is also important for handling considerations whether the toe angle, which is normally set when the vehicle is stationary, changes with speed, roll, pitch or wheel jounce and rebound. Roll is the rotation of the vehicle body about a longitudinal axis of the vehicle, such as is induced during sharp cornering, especially with very soft suspension rates. Pitch is rotation of the body about the lateral axis of the vehicle, such as is induced by heavy braking or acceleration. Jounce is the relative displacement of the wheel upward toward the body from the static condition, typically compressing the suspension springs, while rebound is the relative displacement of the wheel downward, away from the body, from the static condition.
The geometry of a suspension linkage determines a wheel's static toe angle and how it changes with wheel travel. The length and position of a toe control link are primarily responsible for the final shape of the “toe angle vs. wheel travel” curve. It is generally understood that incorporating deliberate toe change as a function of wheel travel can offset other ride-induced handling effects. For example, on many rear suspensions the wheel on the outside of a bend will tend to go into jounce and be pushed into toe-out by lateral cornering forces. This creates an oversteer effect that can lead to a disconcerting and potentially dangerous overswing of the rear of the vehicle. To compensate for this effect, the toe control link may be configured to effect a toe-in on jounce (such as will be experienced by the outside wheel on cornering). As a reference point, a stock 1994 LEXUS LS400 rear suspension is believed to be configured to have about 0.6 degrees of toe-in at about 80 millimeters of jounce travel.
On front suspensions, cornering forces generally tend to straighten the turned-ill wheel on the outside of a curve, slightly offsetting steering input and causing a safe and predictable understeer condition. Introducing geometric toe-out with jounce will tend to increase understeer in a front suspension.
The tread width for a given pair of wheels is defined as the lateral distance between the center of tire contact with the road. When wheels jounce and rebound on most practical independent suspensions for passenger cars, the tread width changes. Solid axle suspensions (generally not favored due to ride quality and weight issues) and full trailing arm suspensions (generally considered only for rear suspensions on straight line racing vehicles, such as for drag racing), are generally immune from tread width changes. On preferred passenger vehicle independent suspensions, such as the strut and SLA suspensions, the rather short suspension links pivot about fixed points or axes on the body, inboard of the wheel assemblies. The outer ends of the links (attached to the wheel assemblies) are therefore constrained to move in a substantially circular path with respect to the body, as viewed from either end of the vehicle. This link motion alters the position of the bottom of the tire relative to the body, changing the overall tread width.
Tread width change creates lateral forces, higher rolling resistance, and deterioration in directional stability of the vehicle. Conventional passenger cars typically have tread widths that widen with jounce travel and narrow during rebound. When a bump compresses both wheels of an independent axle simultaneously, the lateral forces applied to the vehicle body by lateral movement of one wheel tend to be balanced by lateral forces from the other wheel. Unfortunately, bumps seldom generate equal jounce and rebound on both sides of the vehicle simultaneously, and unequal wheel motions result in net lateral forces being applied to the vehicle body from tread width changes. As a reference point, a single 175/65 R 14 radial tire is believed to create about 30 Newtons of lateral force per 1 millimeter of tread width change at 80 kilometers per hour (kph).
As undesirable as tread width change can be, configuring a conventional suspension to geometrically eliminate tread width changes tends to create unacceptable levels of roll during cornering, due to positioning of the theoretical “roll centers” of the front and rear suspensions. An excessive amount of roll is uncomfortable to the driver and passengers, can adversely affect tire grip, and uses up valuable suspension travel needed to avoid bottoming of the suspension on bumpy corners. The suspension roll center is a theoretical point in the center of the vehicle (viewed from the front) and in the center of the axle (viewed from the side) around which the vehicle body will rotate when subject to centrifugal force. It is also the point at which lateral forces can be viewed as effectively applied to the sprung body mass by the suspension. Therefore, the tendency for the vehicle body to roll is proportional to the distance between the roll center and the center of gravity of the body, and the optimum position of the roll center to minimize roll is at the height of the center of gravity of the sprung mass. However, the higher the roll center, the larger the tread width change. With the roll center above ground level, tread width will increase during jounce and decrease on rebound. It will be understood that the location of the roll center constantly changes with suspension position.
Wheel camber is another variable for tuning the characteristics of a vehicle suspension. Camber is the angle between the wheel center plane and a vertical to the plane of the road. Camber is positive when the top of the tire is inclined outwards away from the center of the vehicle, and negative when inclined inwards. When a vehicle is loaded to its design weight, a slightly positive camber value of, for example, 0.1 degree is considered ideal to keep the tires as upright as possible on the crowned road surface, for low rolling resistance and uniform tire wear. Many passenger car suspensions employ a static camber setting between about zero and negative 1.3 degrees, and effect dynamic camber change through selective suspension compliance, to offset negative static camber to try to provide a nearly zero camber on the outside tires in cornering. Some have said that keeping the camber near zero under all conditions is a primary goal of modern suspension systems, perhaps because tires are all designed to operate at a particular camber angle for optimum grip, and even small deviation from that angle can reduce tire grip capability.
When a vehicle with independent suspension is cornering, the wheels tend to tilt with the body. Thus, as the car body rolls toward the outside of the bend, the outside wheel goes into positive camber relative to the road, reducing its lateral grip. To combat this effect, many suspension linkages are designed to geometrically induce negative camber in jounce and positive camber as they rebound, even though such geometric camber adjustments will cause camber shifts during bumps as well as during cornering.
Cars with relatively soft suspensions will tend to pitch during braking and acceleration, dipping at the front and rising at the rear under heavy braking, and the opposite during hard acceleration. This pitching motion tends to put more strain on the neck muscles of the vehicle occupants than during simple linear acceleration and deceleration without such body rotation. Also, pitching motions are perceived to be objectionable by many passengers. Many suspensions incorporate anti-dive (to reduce forward pitching during braking) and anti-squat (to reduce rearward pitching during acceleration) configurations to reduce this effect.
An example of a simple anti-dive design is the use of a leading arm in the front suspension and a trailing arm in the rear suspension. With a leading arm, the effective arm pivot at the body is rearward of the effective arm pivot at the wheel. The relative locations of the effective pivots is reversed for a trailing arm. Under braking action, the calipers tend to rotate with the wheel producing an upward reaction at the front of the body and a downward force at the rear, producing an anti-dive effect. Traditional suspension linkages, like the double wishbone, can be designed with the wishbones pivot axes angled to give an effective leading arm length. However, such arrangements tend to induce undesirable wheel caster angle changes during jounce and rebound. Therefore, some manufacturers compromise by correcting only a percentage (typically, about 50 percent) of the brake live. Furthermore, an anti-dive geometry carefully calculated to match a particular front/rear brake force distribution will seldom give the perfect correction for anti-squat.
Improvements in suspension configuration are generally needed, particularly for use with active suspension control means.